The effects of suspended sediments on the physiology and performance of coral reef fishes

Abstract

Declining water quality, in particular increasing suspended sediment concentrations, is a major factor causing the degradation of marine ecosystems. Anthropogenic activities such as coastal development, agriculture, cattle grazing, mining, dredging, and shipping have led to an increase in suspended sediments in coastal waters over the past decades. On coral reefs, suspended sediments have been linked to declines in species diversity and abundance of both benthic and reef fish communities. Changes in physiological performance have been hypothesized to represent a mechanism driving the observed declines in fish populations, but evidence for this is currently lacking. Moreover, the cumulative effects of suspended sediments and co-occurring environmental factors, such as elevated ocean temperatures caused by anthropogenic climate change and different water flow regimes, on coral reef fishes are unknown. This information is essential to understand responses of reef fish populations to current and future environmental conditions. The overall aim of this thesis was to examine the effects of suspended sediments, both in isolation and in combination with elevated water temperature and elevated water flow, on the physiology and performance of coral reef fishes. Suspended sediments commonly induce changes in fish gill morphology that have been hypothesized to compromise gill function. In chapter two, the effects of suspended sediments on the gill morphology and aerobic performance of three damselfish species (Amphiprion melanopus, Amphiprion percula, and Acanthochromis polyacanthus) were examined. Following suspended sediment exposure (0, 45, 90, 135 or 180 mgL⁻¹), all three species exhibited reductions in the length of gill lamellae (a proxy for gill surface area) and/or reductions in gas diffusion distances. Yet, only A. melanopus exhibited changes in oxygen uptake rates, i.e., a decrease in maximum oxygen uptake rates (ṀO₂max) and an increase in resting oxygen uptake rates (ṀO₂rest). This resulted in a decreased aerobic scope in A. melanopus, indicative of a reduced capacity for aerobic activities, such as growth and locomotion. Results from this chapter indicate that A. melanopus and other species that exhibit a reduction in aerobic performance induced by suspended sediments may decline as reefs become more turbid. In contrast, species that are able to maintain aerobic performance despite changes in gill morphology, such as A. polyacanthus and A. percula, may be able to persist or even gain a competitive advantage on turbid reefs. The ability to avoid and escape predators is key to the survival for juvenile coral reef fishes. If fish undergo changes in gill morphology that result in decreased aerobic scope, as observed in chapter two, suspended sediments may potentially compromise predator escape performance and influence anti-predator behaviours (i.e., behaviours that reduce the likelihood of encountering a predator, such as vigilance, changes in activity levels and sheltering). In chapter three, juvenile A.…